An electrolytic cell is a layered structure that has anode and cathode electrode layers sandwiching an electrolyte layer that is formed of a solid oxide such as yttria stabilized zirconia or gadolinium doped ceria. When an electrical potential is applied across to the electrodes, oxygen, in an oxygen containing feed, ionizes to produce oxygen ions which are transported through the electrolyte. The oxygen ions emerge from the electrolyte and recombine to form elemental oxygen.
Electrolytic cells can be in the form of flat plates or tubes. The electrodes and electrolyte can be supported on an inert supporting structure such as alumina or the electrode serving as the anode can be configured to provide the support. Electrolytic cells are formed by either applying the anode layer to the inert support or forming the anode layer into the desired shape of the electrolytic cell, for instance, a tube. Thereafter, the electrolyte layer is applied to the anode layer and the structure is fired to burn out binders and to sinter the ceramic and thereby to produce a sintered intermediate form. A cathode layer located opposite to the anode is formed by applying a green cathode layer onto the electrolyte layer. The resultant structure is further fired to sinter the cathode layer. The various layers can be formed and applied by known techniques such as extrusion, tape casting, and slurry dipping.
The anode and cathode layers can be formed of metallic, electrically conductive materials, mixtures of mixed conductors (materials that conduct both oxygen ions and electrons), ionic conductors (materials that solely conduct oxygen ions) and metallic conductors. Typical mixed conductors are various known perovskites and typical purely ionic conductors are yttria-stabilized zirconia or gadolinium doped ceria.
The application of the electrolyte layer and the cathode layer is extremely important in the fabrication of the electrolytic cell. If the electrolyte layer has defects, that extend through the electrolyte layer, such as pin holes, the material forming the cathode can penetrate such defects to provide an electrical pathway between the cathode and the anode. This electrical pathway, known as a “short”, will make the electrolytic cell non-functional.
In current manufacturing technologies, electrolytic cells are prone to shorting after sintering. In fact, manufacturing yields can be quite low, less than 40%. The reason for the low productivity is believed to be related to the fact that the dense electrolyte film is typically very thin, about 25 microns, and cannot sinter to 100% of its theoretical density using current available technology. The electrolyte layer more or less will contain a certain degree of defects which can be of a sub-micron scale. The production of the cathode layer, particularly when applied by way of a slurry solution, utilizes the application of fine particles that are of the same scale as defects existing within the electrolyte. Such fine particles are able to penetrate the defect and thereby produce the short between the cathode and the anode.
The prior art has attempted to solve this problem in different ways. In one example, disclosed in U.S. Pat. No. 5,358,735, the electrolyte layer is applied to a porous anode layer by thermal spraying. The electrolyte is then impregnated with a solution of a metal compound consisting of at least one metal selected from the group of magnesium, iron, cobalt, nickel, copper and zinc. In U.S. Pat. No. 5,085,742, a solid oxide fuel cell fabrication is disclosed in which the electrolyte layer is plasma sprayed. Defects are plugged using an EVD coating. The plugs form due to a reaction between vapors feeding from the electrolyte side and oxygen feeding from the porous support side. U.S. Pat. No. 6,165,553 discloses spraying a liquid polymer precursor on a thin-film surface. The liquid is selectively wicked into open pores of the film to effectively plug pores that would otherwise exist within the electrolyte layer. The treated sample is then heated to high temperature to create solid plug in the pores of the film.
Plugs formed by impregnation of a metal solution or special precursor into pores of the electrolyte can leave gaps within the electrolyte due to shrinkage during firing. As a result, defects will be produced by the resultant gaps. EVD processing has the disadvantage of taking time and thereby decreasing production rate. Additionally, EVD adds expense to the production by increasing the initial capital investment involved in obtaining the necessary equipment.
As will be discussed, the present invention provides a simple process that can be used to cure detects within the electrolyte layer that would otherwise lead to shorting that is more effective than the prior art and can be applied by techniques that are less involved and less expensive than those of the prior art.